Interview by Kal Kaur
Dr. Martin Maldovan, a researcher at the Department of Materials Science and Engineering at the Massachusetts Institute of Technology, talks to Kal Kaur from AZoNano about his work on heat manipulation with semiconductor alloy crystals.
KK: Can you provide a summary of the theoretical principle behind your research on a technique for manipulating heat?
MM: Both heat and sound are atomic vibrations transmitted through matter. One of the main differences, however, is that sound vibrates at low-frequencies (~kHz) and travels large distances, while heat vibrates at high-frequencies (~THz) and travels short distances. In addition, heat is made of a relatively large range of frequencies. The theoretical principle behind the research is that, if we suppress the high-frequency vibrations that contribute to heat transfer, the remaining lower frequency vibrations can be treated like sound. This result can have a significant impact for thermal management as materials and devices already developed to manipulate sound (e.g., phononic crystals) could be now employed to control heat.
KK: Heat can span a large spectrum of frequencies, how will your new technique to manipulate heat address this issue?
MM: Yes, heat can span a large range of frequencies and this is why it’s so difficult to control. The basic idea in this novel way to control heat conduction is based on the following two concepts: a) to transform heat such that it is carried by low-frequency vibrations, b) to narrow the number of frequencies capable to carry heat.
KK: What process is involved in reducing this wide range of frequencies for heat to a narrow range that can be manipulated?
MM: To reduce the number of heat frequencies, I kill high-frequency contributions by including alloy atoms and small nanoparticles (~1nm). These little imperfections in the atomic lattice substantially reduce high-frequency vibrations without altering low-frequency vibrations. In addition, I use boundary size effects to effectively kill very-low frequency phonons. This can be done by adjusting the thickness of the films. After eliminating high- and very low-frequency contributions, the net result is that a substantial part of the heat is carried by a narrow low-frequency band.
KK: Why is it important to create a state of “hypersonic heat”, and how are nanomaterials used to achieve this?
MM: “Hypersonic heat” refers to these atomic vibrations that can carry heat but they have lower frequencies than “standard” heat in semiconductor materials. At the same time, these vibrations have higher frequencies than sound. And that is why I use the term “hypersonic heat”. The vital importance of these vibrations is the fact that they carry heat but they can be controlled as sound.
Thermal lattices, shown here, are one possible application of the newly developed thermocrystals. In these structures, where precisely spaced air gaps (dark circles) control the flow of heat, thermal energy can be "pinned" in place by defects introduced into the structure (colored areas). Image courtesy of Martin Maldovan.
KK: How does the control of heat compare to the manipulation of light waves by lenses and mirrors?
MM: “Standard” heat is made of high-frequency (short-wavelength) vibrations, and when these vibrations are incident on typical interfaces, they generally scatter diffusely along any direction. In contrast, “hypersonic heat” vibrations have lower frequencies (and consequently larger wavelengths) than standard heat. Due to their larger wavelengths, when “hypersonic heat” vibrations are incident on typical interfaces, they can be subject to reflection and transmission similarly to light in lenses and mirrors.
KK: How much heat flow can you concentrate within the hypersonic range using this new technique?
MM: By engineering the heat spectrum, I was able to concentrate most of the heat spectra to a relatively low-frequency window 0.1THz-2.0THz, with up to 40% of the heat concentrated to a narrow “hypersonic” 100-300 GHz range.
KK: What challenges were you faced with when developing this technique to manipulate heat?
MM: The challenge was to find suitable mechanisms to lower the frequencies for heat vibrations and to narrow the frequency spectrum such that the resultant frequency band was consistent with frequencies of phononic crystals. These types of crystals are periodic structures made of two elastic materials where vibrations with wavelengths comparable to the periodicity can be controlled in many useful ways due to forbidden frequency gaps. The basic idea is that, if a phononic band gap is put at heat carrying frequencies, interesting thermal effects could be obtained. The results show that rationally designed phononic crystals with periodicities 10-20nm can be used to manage heat carrying vibrations.
KK: What further developments, changes to the technique and material used will be involved in increasing the precision and accuracy of this new technique?
MM: There are still challenges ahead in terms of increasing the efficiency of this new material. To be able to efficiently manipulate hypersonic vibrations by using sonic materials such as phononic crystals, the range of frequencies for heat transfer should be narrowed further. Additional improvements in this direction can be achieved by using impurities, dislocations, and/or amorphous materials. Alternatively, the current search for material structures with large sonic band gaps can help to increase the frequencies that phononic crystals can control. Therefore, there exist two independent directions for increasing the efficiency of these new materials for heat conduction.
KK: What application areas will this technique be suited to?
MM: The precise control of heat is critical in the development of highly-efficient thermoelectric materials, which convert waste heat into useful electricity. This new heat transfer technique can help to further increase the effectiveness of thermoelectrics. Additionally, by using this technique, heat can be channelled along waveguides providing a platform for small scale thermal circuits. One other long-term application is the possibility of creating thermal diodes, which are systems where heat vibrations can be transmitted along a given direction but not in the reverse direction, in analogy with electrical circuits.
KK: What are the advantages of this new technique over standard methods to control heat in these applications?
MM: This technique is a whole new way to manipulate heat. Heat vibrations are not scattered diffusely at interfaces, instead, they are reflected and transmitted. In comparison with other methods, this technique can allow to control thermal flow with good precision, offering the possibility of initiating significant new developments by creating novel thermal effects and devices.
KK: Where can we find more information?
MM: More information about these findings can be found in the article published in Physical Review Letters, Jan 11, 2013.
About Dr. Martin Maldovan
Martin Maldovan is a Researcher in the Department of Materials Science and Engineering at the Massachusetts Institute of Technology in Cambridge, Massachusetts, USA.
He received his B.S. in Physics from the University of Buenos Aires, Argentina, and his M.S. and Ph.D. in Materials Science from the Massachusetts Institute of Technology. Dr. Martin Maldovan has authored numerous scientific publications in the fields of photonics, phononics and mechanics and obtained the 2006 Scientific Writing Award to Professionals in Acoustics from the Acoustical Society of America.
His research interests include the prediction of material properties and rational design of materials by computational simulations, wave-matter interactions, merging photonics and phononics for novel light-sound interaction, and the development of new thermal materials for energy applications.